The present invention relates to an apparatus for injecting a liquid dye into a polymer melt of the general type disclosed in DE 199 56 251 A1.
To obtain a coloration of the polymer melt when melt spinning synthetic multifilament yarns, it is common practice in the known device to add a liquid dye to the polymer melt and to mix it in a subsequent step. To this end, the dye is stored in a tank. Connected to the tank is a metering pump. The metering pump connects via a line to a spin pump that advances the polymer melt. The spin pump receives the polymer melt via an extruder. To dye the polymer melt, the metering pump adds the liquid dye to the polymer melt in measured quantities.
To obtain over time a uniform and constant coloration of the polymer melt, it is a prerequisite that the liquid dye be supplied to the polymer melt continuously in an exact quantity. However, with the known device there arises the problem that the metering pump must bridge a pressure difference between the liquid dye that is kept under no pressure, and the melt that is supplied under an overpressure. For example, at the outlet of an extruder, the polymer melt advances under an overpressure of about 100 bars. With that, however, the metering pump incurs to an increasing extent volumetric losses, which have a negative effect on a quantity adjustment for metering the dye. A further problem of the known device lies in that the direct connection between the spin pump and the metering pump causes a pressurized polymer melt to flow off directly when the metering pump is kept under no pressure.
It is therefore an object of the invention to further develop an apparatus of the initially described type as well as a method of the initially described type in such a manner that it is possible to add to a polymer melt a liquid dye as accurately metered as possible.
It is a further object of the invention to provide an apparatus of the described type which can be adapted with great flexibility to a spin unit or another melt carrying unit for injecting a liquid dye or liquid additives.